# 6.1 Graphs of the sine and cosine functions  (Page 6/13)

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## Identifying the properties of a sinusoidal function

Given $\text{\hspace{0.17em}}y=-2\mathrm{cos}\left(\frac{\pi }{2}x+\pi \right)+3,\text{\hspace{0.17em}}$ determine the amplitude, period, phase shift, and horizontal shift. Then graph the function.

Begin by comparing the equation to the general form and use the steps outlined in [link] .

$y=A\mathrm{cos}\left(Bx-C\right)+D$
• Step 1. The function is already written in general form.
• Step 2. Since $\text{\hspace{0.17em}}A=-2,\text{\hspace{0.17em}}$ the amplitude is $\text{\hspace{0.17em}}|A|=2.$
• Step 3. $\text{\hspace{0.17em}}|B|=\frac{\pi }{2},\text{\hspace{0.17em}}$ so the period is $\text{\hspace{0.17em}}P=\frac{2\pi }{|B|}=\frac{2\pi }{\frac{\pi }{2}}=2\pi \cdot \frac{2}{\pi }=4.\text{\hspace{0.17em}}$ The period is 4.
• Step 4. $\text{\hspace{0.17em}}C=-\pi ,$ so we calculate the phase shift as $\text{\hspace{0.17em}}\frac{C}{B}=\frac{-\pi ,}{\frac{\pi }{2}}=-\pi \cdot \frac{2}{\pi }=-2.\text{\hspace{0.17em}}$ The phase shift is $\text{\hspace{0.17em}}-2.$
• Step 5. $D=3,$ so the midline is $\text{\hspace{0.17em}}y=3,$ and the vertical shift is up 3.

Since $\text{\hspace{0.17em}}A\text{\hspace{0.17em}}$ is negative, the graph of the cosine function has been reflected about the x -axis.

[link] shows one cycle of the graph of the function.

## Using transformations of sine and cosine functions

We can use the transformations of sine and cosine functions in numerous applications. As mentioned at the beginning of the chapter, circular motion can be modeled using either the sine or cosine function    .

## Finding the vertical component of circular motion

A point rotates around a circle of radius 3 centered at the origin. Sketch a graph of the y -coordinate of the point as a function of the angle of rotation.

Recall that, for a point on a circle of radius r , the y -coordinate of the point is $\text{\hspace{0.17em}}y=r\text{\hspace{0.17em}}\mathrm{sin}\left(x\right),\text{\hspace{0.17em}}$ so in this case, we get the equation $\text{\hspace{0.17em}}y\left(x\right)=3\text{\hspace{0.17em}}\mathrm{sin}\left(x\right).\text{\hspace{0.17em}}$ The constant 3 causes a vertical stretch of the y -values of the function by a factor of 3, which we can see in the graph in [link] .

What is the amplitude of the function $\text{\hspace{0.17em}}f\left(x\right)=7\mathrm{cos}\left(x\right)?\text{\hspace{0.17em}}$ Sketch a graph of this function.

7

## Finding the vertical component of circular motion

A circle with radius 3 ft is mounted with its center 4 ft off the ground. The point closest to the ground is labeled P , as shown in [link] . Sketch a graph of the height above the ground of the point $\text{\hspace{0.17em}}P\text{\hspace{0.17em}}$ as the circle is rotated; then find a function that gives the height in terms of the angle of rotation.

Sketching the height, we note that it will start 1 ft above the ground, then increase up to 7 ft above the ground, and continue to oscillate 3 ft above and below the center value of 4 ft, as shown in [link] .

Although we could use a transformation of either the sine or cosine function, we start by looking for characteristics that would make one function easier to use than the other. Let’s use a cosine function because it starts at the highest or lowest value, while a sine function    starts at the middle value. A standard cosine starts at the highest value, and this graph starts at the lowest value, so we need to incorporate a vertical reflection.

Second, we see that the graph oscillates 3 above and below the center, while a basic cosine has an amplitude of 1, so this graph has been vertically stretched by 3, as in the last example.

Finally, to move the center of the circle up to a height of 4, the graph has been vertically shifted up by 4. Putting these transformations together, we find that

$y=-3\mathrm{cos}\left(x\right)+4$

A weight is attached to a spring that is then hung from a board, as shown in [link] . As the spring oscillates up and down, the position $\text{\hspace{0.17em}}y\text{\hspace{0.17em}}$ of the weight relative to the board ranges from $\text{\hspace{0.17em}}–1\text{\hspace{0.17em}}$ in. (at time $\text{\hspace{0.17em}}x=0\right)\text{\hspace{0.17em}}$ to $\text{\hspace{0.17em}}–7\text{\hspace{0.17em}}$ in. (at time $\text{\hspace{0.17em}}x=\pi \right)\text{\hspace{0.17em}}$ below the board. Assume the position of $\text{\hspace{0.17em}}y\text{\hspace{0.17em}}$ is given as a sinusoidal function of $\text{\hspace{0.17em}}x.\text{\hspace{0.17em}}$ Sketch a graph of the function, and then find a cosine function that gives the position $\text{\hspace{0.17em}}y\text{\hspace{0.17em}}$ in terms of $\text{\hspace{0.17em}}x.$

$y=3\mathrm{cos}\left(x\right)-4$

I've run into this: x = r*cos(angle1 + angle2) Which expands to: x = r(cos(angle1)*cos(angle2) - sin(angle1)*sin(angle2)) The r value confuses me here, because distributing it makes: (r*cos(angle2))(cos(angle1) - (r*sin(angle2))(sin(angle1)) How does this make sense? Why does the r distribute once
How can you tell what type of parent function a graph is ?
generally by how the graph looks and understanding what the base parent functions look like and perform on a graph
William
if you have a graphed line, you can have an idea by how the directions of the line turns, i.e. negative, positive, zero
William
y=x will obviously be a straight line with a zero slope
William
y=x^2 will have a parabolic line opening to positive infinity on both sides of the y axis vice versa with y=-x^2 you'll have both ends of the parabolic line pointing downward heading to negative infinity on both sides of the y axis
William
y=x will be a straight line, but it will have a slope of one. Remember, if y=1 then x=1, so for every unit you rise you move over positively one unit. To get a straight line with a slope of 0, set y=1 or any integer.
Aaron
yes, correction on my end, I meant slope of 1 instead of slope of 0
William
what is f(x)=
I don't understand
Joe
Typically a function 'f' will take 'x' as input, and produce 'y' as output. As 'f(x)=y'. According to Google, "The range of a function is the complete set of all possible resulting values of the dependent variable (y, usually), after we have substituted the domain."
Thomas
Sorry, I don't know where the "Â"s came from. They shouldn't be there. Just ignore them. :-)
Thomas
Darius
Thanks.
Thomas
Â
Thomas
It is the Â that should not be there. It doesn't seem to show if encloses in quotation marks. "Â" or 'Â' ... Â
Thomas
Now it shows, go figure?
Thomas
what is this?
i do not understand anything
unknown
lol...it gets better
Darius
I've been struggling so much through all of this. my final is in four weeks 😭
Tiffany
this book is an excellent resource! have you guys ever looked at the online tutoring? there's one that is called "That Tutor Guy" and he goes over a lot of the concepts
Darius
thank you I have heard of him. I should check him out.
Tiffany
is there any question in particular?
Joe
I have always struggled with math. I get lost really easy, if you have any advice for that, it would help tremendously.
Tiffany
Sure, are you in high school or college?
Darius
Hi, apologies for the delayed response. I'm in college.
Tiffany
how to solve polynomial using a calculator
So a horizontal compression by factor of 1/2 is the same as a horizontal stretch by a factor of 2, right?
The center is at (3,4) a focus is at (3,-1), and the lenght of the major axis is 26
The center is at (3,4) a focus is at (3,-1) and the lenght of the major axis is 26 what will be the answer?
Rima
I done know
Joe
What kind of answer is that😑?
Rima
I had just woken up when i got this message
Joe
Rima
i have a question.
Abdul
how do you find the real and complex roots of a polynomial?
Abdul
@abdul with delta maybe which is b(square)-4ac=result then the 1st root -b-radical delta over 2a and the 2nd root -b+radical delta over 2a. I am not sure if this was your question but check it up
Nare
This is the actual question: Find all roots(real and complex) of the polynomial f(x)=6x^3 + x^2 - 4x + 1
Abdul
@Nare please let me know if you can solve it.
Abdul
I have a question
juweeriya
hello guys I'm new here? will you happy with me
mustapha
The average annual population increase of a pack of wolves is 25.
how do you find the period of a sine graph
Period =2π if there is a coefficient (b), just divide the coefficient by 2π to get the new period
Am
if not then how would I find it from a graph
Imani
by looking at the graph, find the distance between two consecutive maximum points (the highest points of the wave). so if the top of one wave is at point A (1,2) and the next top of the wave is at point B (6,2), then the period is 5, the difference of the x-coordinates.
Am
you could also do it with two consecutive minimum points or x-intercepts
Am
I will try that thank u
Imani
Case of Equilateral Hyperbola
ok
Zander
ok
Shella
f(x)=4x+2, find f(3)
Benetta
f(3)=4(3)+2 f(3)=14
lamoussa
14
Vedant
pre calc teacher: "Plug in Plug in...smell's good" f(x)=14
Devante
8x=40
Chris
Explain why log a x is not defined for a < 0
the sum of any two linear polynomial is what
Momo
how can are find the domain and range of a relations
the range is twice of the natural number which is the domain
Morolake
A cell phone company offers two plans for minutes. Plan A: $15 per month and$2 for every 300 texts. Plan B: $25 per month and$0.50 for every 100 texts. How many texts would you need to send per month for plan B to save you money?
6000
Robert
more than 6000
Robert
For Plan A to reach $27/month to surpass Plan B's$26.50 monthly payment, you'll need 3,000 texts which will cost an additional \$10.00. So, for the amount of texts you need to send would need to range between 1-100 texts for the 100th increment, times that by 3 for the additional amount of texts...
Gilbert
...for one text payment for 300 for Plan A. So, that means Plan A; in my opinion is for people with text messaging abilities that their fingers burn the monitor for the cell phone. While Plan B would be for loners that doesn't need their fingers to due the talking; but those texts mean more then...
Gilbert